Greenwich Observatory

It depends on what datum you are using. The Greenwich meridian is about 100m different from the meridian used by the WGS84 datum, which is what GPS uses as standard. And they are slowly moving relative to each other.
If you set your GPS to the Ordnance Survey GB datum, then the meridian will be pretty close to the Greenwich meridian (apparently about 6m different).

Some more details on Wikipedia: http://en.wikipedia.org/wiki/Prime_meridian

The Greenwich meridian is about 100m different from the meridian used by the WGS84 datum, which is what GPS uses as standard. And they are slowly moving relative to each other.

Thank you very much. I didn't realize there was so much difference in the "standards" for such things. In fact, it's (or was when I last checked) about 335 feet from the so-called prime meridian to what my GPS thinks is 0 degrees, which is near as dammit to 100 meters. The link was interesting as well.

I realize this topic isn't espcially BC-specific, but there may be other frustrated old-school navigators out there.

The research that I've done as a result of the answer I got here revealed dozens of different "standards" for coordinate systems. Who knew? Certainly not I.

In any event, if I had a pile of nautical charts from the '70s (which I would still have, if they hadn't been stolen from my boat), and I calculated a lat/long location using them, would that value be 335 feet different than what I would get with my GPS? If so, it's clear that navigation has certainly moved into what I guess would best be considered a new dimension.

I used to think that longitude was longitude, but apparently not. There apparently needs to be a note somewhere on the chart that references the "standard," much like the magnetic variation. Oh wait, no one uses magnetic compasses anymore, do they?

The research that I've done as a result of the answer I got here revealed dozens of different "standards" for coordinate systems. Who knew? Certainly not I.

The nice thing about standards is that you have so many to choose from.

Andrew S. Tanenbaum, Computer Networks, 2nd ed., p. 254

Well in the GPS world the WGS84 datum is the norm. However many paper maps use other datums and/or coordinate systems. Fortunately BaseCamp/MapSource/etc allow you to select both. Often the issue is trying to figure out which ones were used.

BTW you could always go back to a marine chronometer and a sextant :D

BTW you could always go back to a marine chronometer and a sextant :D

Well, mock it if you will, but that's exactly what I'm asking about. My earlier comment regarding the sun, stars and a clock was a reference to how these sorts of things were done for centuries. I'm interested in learning if the answer I get that way (really old school) gives the same answer as a GPS that can be acquired by any moron with a hundred dollars or so.

Navigation to me is something of a religion, and I find it troubling that being a hundred yards off isn't considered a problem. I'm guessing that the number of people using a GPS who also know what the Nautical Alminac is (was) and how to use HO229 sight reduction tables could fit in a small space.

Go figure.

Navigation to me is something of a religion, and I find it troubling that being a hundred yards off isn't considered a problem.

It is a problem in many cases (e.g. marine navigation). I'm not sure how "old school" navigators handled different datums. My guess is that there was only one (that they used) back then. With GPS if you're using the "correct" datum you should be good. I'd imagine that these days almost everyone would use WGS84. The Greenwich Prime Meridian was defined way before GPS so I doubt it "being off" is a practical issue these days.

I don't know if you've ever heard of Selective Availability but until Clinton turned it off in 2000 civilian GPS had a deliberate error of up to 100m injected making it almost useless for navigation.

At sea when when on the surface and using old-school nav devices I would be very happy if I was only 335 feet off from where I thought I might have been when I took the fix. Near the coast it was mostly visual or radar fixes which served to eliminate any datum choice problems. If the waters were really tricky then that's what channel markers and Pilots are for.

Try turning off the "snap to road" function on your devices and you may be surprised...

Yeah. Bummer when it's foggy and/or overcast. "Old school" doesn't work too well then ;)

Well, mock it if you will, but that's exactly what I'm asking about. My earlier comment regarding the sun, stars and a clock was a reference to how these sorts of things were done for centuries. I'm interested in learning if the answer I get that way (really old school) gives the same answer as a GPS that can be acquired by any moron with a hundred dollars or so.

Navigation to me is something of a religion, and I find it troubling that being a hundred yards off isn't considered a problem. I'm guessing that the number of people using a GPS who also know what the Nautical Alminac is (was) and how to use HO229 sight reduction tables could fit in a small space.

Go figure.

You might like to pop into here http://micronavigation.com/forum/index.php for some serious navigation talk!

Well, mock it if you will, but that's exactly what I'm asking about. My earlier comment regarding the sun, stars and a clock was a reference to how these sorts of things were done for centuries. I'm interested in learning if the answer I get that way (really old school) gives the same answer as a GPS that can be acquired by any moron with a hundred dollars or so.

Of course. ... As long as the moron with the GPS makes sure s/he has the proper maps/charts loaded into the GPS.

Navigation to me is something of a religion, and I find it troubling that being a hundred yards off isn't considered a problem. I'm guessing that the number of people using a GPS who also know what the Nautical Alminac is (was) and how to use HO229 sight reduction tables could fit in a small space.

Go figure.

Well, as with most anything, there are [at least] two sides to this. One is that anyone navigating on water away from land better take proper on-water navigation religiously. That means they need to understand the tools they choose to use; their strengths and their limitations and how to use them competently.

The other is that with the proper charts and either a GPS that supports them or a chart plotter, the issue of finding current location becomes rather trivial, as you should expect.

What's left is understanding what to do with that knowledge. :cool:

I took one of those vacation cruises just over a year ago, from Newark, NJ, to the Caribbean. For grins I loaded Garmin's Bluecharts onto my netbook and took along my Garmin 10x Bluetooth GPS puck. Whenever I was able to verify the location with this setup it was right on the mark. That was either through visual verification whenever we were near land or from the data on one of the TV channels in our stateroom that ran 24/7.

That doesn't qualify me to navigate in anything larger than the mud puddle that develops in front of my driveway each spring. But it significantly reduces the technical difficulty of determining my location at any given time.

As long as the batteries don't run out. :eek:

...ken...